Perhaps it's some RAM from a while back that someone found, identified it to be yours and mailed it back. Or, it could be a dead-letter forwarded on by whoever found it in the bottom of a mailbag somewhere. Or it could be returned by someone who found the RAM after you had sold it to someone else, but they had not used it and passed away.

I could keep going, but you can see the multitude of possibilities by this point, I'm sure.